This invention relates to a hydraulic booster device used in a brake system of vehicles to assist a driver's brake operating force, and more particularly, to an improved technique of the type in which a servo fluid pressure is generated by the effect of a throttle valve provided within the device.
In a hydraulic booster device of this type, a servo fluid pressure is generated by the effect of the throttle valve and therefore, there is no need of a provision of an accumulator or the like, for reserving a pressure-increased fluid pressure within the system including the device. Because of this reason, the device of this type is advantageous in miniaturizing the overall system. Moreover, in the system including the booster device of this type, a working fluid supplied by a pump circulates within the closed system so that it will return to the pump after it flows into the booster device. Owing to such an arrangement, in the system including a booster device of this type, the working fluid flowed out of the booster device is, in many cases, utilized to actuate other devices as represented by a power steering device. By commonly using a single pump in the booster device, in the power steering device and the like, the overall system can be more simplified. For more details, reference should be made, for example, to Japanese Utility Model Unexamined Publication No. Sho 58-35457 or Japanese Patent Examined Publication No. Hei 3-2700.
In a booster device of this type, a main component of valve means for generating the throttling effect is a spool. In response to an input made by a driver, a very tiny clearance is formed between a land portion of an outer periphery of the spool and an inner peripheral wall of a valve bore to which the spool is fitted, the clearance restricts the flow of the working fluid and as a result, a throttling effect occurs. When this throttling effect works, one side of the valve means acting as a throttle valve, i.e., the inlet port side connected to a pump, becomes such high in pressure as exceeding, for example, 120 kg/cm.sup.2, whereas the outlet port side communicating with a power steering device, etc., is very low in pressure. It is made clear that due to effect of the large pressure difference between a forward portion and a backward portion of the valve means, a foreign noise tends to occur from the valve means portion especially when the driver keeps operating the booster device (i.e., when the driver keeps depressing the pedal to maintain the valve means in its throttling state). Since such a noise often gives an uncomfortable feel to the driver, it is demanded to prevent an occurrence of such a noise or to reduce such a noise.
In one attempt to prevent an occurrence of a noise from the valve means portion, a pressure difference between a forward portion and a backward portion of the valve means is reduced by varying the effect of a pump or a throttle. However, in this attempt, a servo pressure generated in a servo chamber drops to reduce the servo ratio of the booster device. Accordingly, in order to obtain a predetermined servo ratio, the diameter of the servo piston must be enlarged. This gives rise to another problem in that the requirement for making the booster device small in size cannot be met.